I'd put a direct reading gauge in the sending unit NPT port in the top rear of the block & drill prime it & see what you DO have. If not enough psi: pipe plug loosened (not likely) wasn't installed (not likely given your level of expertise),lifter popped out (you probably know it if it did) oil pump/regulator problem (again not too likely).If pressure IS OK there I'd take off the rocker shaft assy's and have a helper slowly turn the crank w a breaker bar until he hits the sweet spot as you man the drill and having something over the pedestals that have the passages as it will gush. No gushing points to as said cam bearing or related internal passage to the bearing or the passage from the bearing up to the heads including the head gasket. haven't done it but might insert a straight rod into the pedestal down to the cam bearing & see if you feel any obstruction. But I got a feeling that by this time something will be apparent. Holler when you find what it is.